This invention relates to thermally-assisted magnetic recording (TAMR) disk drives, in which data is written while the magnetic recording layer is at an elevated temperature, and more particularly to a TAMR disk that has a ferromagnetic recording layer exchange-coupled to an antiferromagnetic-to-ferromagnetic switching layer.
Magnetic recording disk drives use a thin film inductive write head supported on the end of a rotary actuator arm to record data in the recording layer of a rotating disk. The write head is patterned on the trailing surface of a head carrier, such as a slider with an air-bearing surface (ABS) to allow the slider to ride on a thin film of air above the surface of the rotating disk. The write head is an inductive head with a thin film electrical coil located between the poles of a magnetic yoke. When write current is applied to the coil, the pole tips provide a localized magnetic field across a gap that magnetizes regions of the recording layer on the disk so that the magnetic moments of the magnetized regions are oriented into one of two distinct directions. The transitions between the magnetized regions represent the two magnetic states or binary data bits. The magnetic moments of the magnetized regions are oriented in the plane of the recording layer in longitudinal or horizontal recording, and perpendicular to the plane in perpendicular or vertical recording.
The magnetic material (or media) for the recording layer on the disk is chosen to have sufficient coercivity such that the magnetized data bits are written precisely and retain their magnetization state until written over by new data bits. The data bits are written in a sequence of magnetization states to store binary information in the drive and the recorded information is read back with a use of a read head that senses the stray magnetic fields generated from the recorded data bits. Magnetoresistive (MR) read heads include those based on anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR) such as the spin-valve type of GMR head, and more recently magnetic tunneling, such as the magnetic tunnel junction (MTJ) head. Both the write and read heads are kept in close proximity to the disk surface by the slider""s ABS, which is designed so that the slider xe2x80x9cfliesxe2x80x9d over the disk surface as the disk rotates beneath the slider.
The areal data density (the number of bits that can be recorded on a unit surface area of the disk) is now approaching the point where magnetic grains that make up the data bits are so small that they can be demagnetized simply from thermal instability or agitation within the magnetized bit (the so-called xe2x80x9csuperparamagneticxe2x80x9d effect). To avoid thermal instabilities of the stored magnetization, a minimal stability ratio of stored magnetic energy per grain, KUV, to thermal energy, kBT, of KUV/kBT greater than  greater than 60 will be required where KU and V are the magneto-crystalline anisotropy and the magnetic switching volume, respectively, and kB and T are the Boltzman constant and absolute temperature, respectively. Because a small number of grains of magnetic material per bit are required to prevent unacceptable media noise, the switching volume V will have to decrease, and accordingly KU will have to increase. However, increasing KU also increases the switching field, H0, which is proportional to the ratio KU/MS, where MS is the saturation magnetization (the magnetic moment per unit volume). (The switching field H0 is the field required to reverse the magnetization direction, which for most magnetic materials is very close to but slightly greater than the coercivity or coercive field HC of the material.) Obviously, H0 cannot exceed the write field capability of the recording head, which currently is limited to about 9 kOe for longitudinal recording, and perhaps 15 kOe for perpendicular recording.
Since it is known that the coercivity of the magnetic material of the recording layer is temperature dependent, one proposed solution to the thermal stability problem is thermally-assisted magnetic recording (TAMR), wherein the magnetic material is heated locally to near or above its Curie temperature during writing to lower the coercivity enough for writing to occur, but high enough for thermal stability of the recorded bits at the ambient temperature of the disk drive (i.e., the normal operating or xe2x80x9croomxe2x80x9d temperature). Several approaches for heating the media in TAMR have been proposed, including use of a laser beam or ultraviolet lamp to do the localized heating, as described in xe2x80x9cData Recording at Ultra High Densityxe2x80x9d, IBM Technical Disclosure Bulletin, Vol. 39, No. 7, July 1996, p. 237; xe2x80x9cThermally-Assisted Magnetic Recordingxe2x80x9d, IBM Technical Disclosure Bulletin, Vol. 40, No. 10, October 1997, p. 65; and IBM""s U.S. Pat. No. 5,583,727. A read/write head for use in a TAMR system is described in U.S. Pat. No. 5,986,978, wherein a special optical channel is fabricated adjacent to the pole or within the gap of a write head for directing laser light or heat down the channel. IBM""s pending application Ser. No. 09/608,848 filed Jun. 29, 2000 describes a TAMR disk drive wherein the thin film inductive write head includes an electrically resistive heater located in the write gap between the pole tips of the write head for locally heating the magnetic recording layer.
Generally, KU and MS of a magnetic material decrease with temperature according to KU(T)xcx9cMS(T)n (e.g., n=3 for cubic materials), such that H0=xcex1KU/MS also decreases steadily with increasing temperature, where xcex1≅1 for isotropic media and xcex1≅2 for highly oriented media. Therefore by heating the media during the write process and letting it cool to room temperature, the write field constraint of the head can be circumvented while at the same time retaining the long time thermal stability of the stored magnetizations representing the recorded data bits. However, for materials with a very high magneto-crystalline anisotropy this requires writing at close to the Curie temperature of the media. In IBM""s pending application Ser. No. 09/874,100 filed Jun. 4, 2001, a technique is described that uses a bilayer of two ferromagnetic materials. The first ferromagnetic layer is formed of a high-coercivity (or magnetically xe2x80x9chardxe2x80x9d) material that has a room temperature coercivity too high for writing with a conventional write head and a low Curie temperature. The second ferromagnetic layer directly above or below the first layer is formed of a low-coercivity (or magnetically xe2x80x9csoftxe2x80x9d) material with a coercivity suitable for writing with a conventional write head and a high Curie temperature. During writing the bilayer is heated to a temperature of about or slightly above the Curie temperature of the first layer, thereby reducing or eliminating the coercivity of the first layer. The bit pattern is then recorded in the second layer. The two layers then cool, and as the first layer cools to below its Curie temperature, it becomes ferromagnetic again and the bit pattern is xe2x80x9ccopiedxe2x80x9d from the second layer into the first layer by magnetic exchange interaction. Upon further cooling the anisotropy of the first layer returns to its original high value, thus providing the desired long-term stability of the recorded data bits.
A practical implementation of this type of TAMR disk uses a layer of low coercivity material suitable for writing with a conventional write head and with a high Curie temperature TCL (e. g., a granular CoPtCrB alloy), and a layer of high coercivity material incapable of being written to by a conventional write head and with a low Curie temperature TCH (e. g., chemically-ordered high anisotropy FePt).
The switching field H0 of such a bilayer material is best approximated by       H    0    =      α    ·                                        K            H                    ⁢                      t            H                          +                              K            L                    ⁢                      t            L                                                            M            H                    ⁢                      t            H                          +                              M            L                    ⁢                      t            L                              
where KH and KL are the anisotropy constants of the high and low coercivity layers, respectively, MH and ML are the saturation magnetization values of the high and low coercivity layers, respectively, and tH and tL are the thicknesses of the high and low coercivity layers, respectively. Writing is then achieved by heating the bilayer to a minimum write temperature TW≅TCH≅600 K, whereby KH is significantly reduced such that H0 of the bilayer is below the available write field of the head. Due to the steep temperature dependence of KH at or near TCH and a distribution of TCH given by the grain size distribution in the high-coercivity layer, it will be desirable to write data at TW greater than TCH, which will undesirably expose adjacent data tracks to thermal decay of the stored information. According to the formula given above, at room temperature the switching field H0 of the bilayer, although higher than the switching field of the low-coercivity layer, H0L=xcex1KL/ML, will always be lower than the switching field of the high-coercivity layer, H0H=xcex1KH/MH. This will effectively reduce the potential gain in areal density provided by the high anisotropy material.
IBM""s U.S. Pat. No. 5,463,578 describes a magneto-optic (MO) recording medium that uses an antiferromagnetic-to-ferromagnetic switching material, such as FeRh, between a high-coercivity bias layer with a very high Curie temperature and the perpendicularly magnetized MO recording layer. The MO layer must be formed of a material with a Curie temperature less than the transition temperature TAF of the switching material (TAF is the temperature at which the material switches from its antiferromagnetic state at room temperature to its ferromagnetic state). To record the MO layer in one magnetization direction, the medium is heated to a temperature above both the Curie temperature of the MO layer and TAF, which renders the switching material ferromagnetic and allows the bias layer to couple its field through the switching material to the MO layer. To record the MO layer in the opposite magnetization direction, the medium is heated to a temperature above the Curie temperature of the MO layer but below TAF so that the switching material remains antiferromagnetic and prevents the bias layer from affecting the magnetization direction of the MO layer. U.S. Pat. No. 5,666,346 describes a MO medium that functions similar to the MO medium of the ""578 patent in that it uses an antiferromagnetic-to-ferromagnetic switching layer to merely mediate coupling between the MO layer and a bias layer. U.S. Pat. No. 5,663,935 describes a MO recording medium that also uses an antiferromagnetic-to-ferromagnetic switching layer beneath the MO layer, but takes advantage of the property that the switching material has a transition temperature hysteresis, meaning that TAF is slightly greater than the temperature TFA at which the material switches from ferromagnetic back to antiferromagnetic. The Curie temperature of the MO layer in the ""935 patent is required to be between TFA and TAF.
For TAMR it is desirable to take full advantage of the room-temperature high anisotropy of a high-coercivity recording material like FePt, but still allow writing to the material at an elevated temperature well below its Curie temperature TC. In addition, this recording material must have sufficient magnetic moment at room temperature to allow reading of the recorded data by a conventional MR read head.
The invention is a TAMR disk that uses a bilayer medium of a high-coercivity high-anisotropy ferromagnetic material like FePt and a switching material like FeRh or Fe(RhM) (where M is Ir, Pt, Ru, Re or Os) that exhibits a switch from antiferromagnetic to ferromagnetic at a transition temperature less than the Curie temperature of the high-coercivity material. The high-coercivity recording layer and the switching layer are exchange coupled ferromagnetically when the switching layer is in its ferromagnetic state. To write data the bilayer medium is heated above the transition temperature of the switching layer. When the switching layer becomes ferromagnetic, the total magnetization of the bilayer is increased, and consequently the switching field required to reverse a magnetized bit is decreased without lowering the anisotropy of the recording layer. The magnetic bit pattern is recorded in both the recording layer and the switching layer. When the media is cooled to below the transition temperature of the switching layer, the switching layer becomes antiferromagnetic and the bit pattern remains in the high-anisotropy recording layer.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.